Cool Stars 14 - Submitted Abstract # 211
This version created on 05 October 2006

Identifying L/T Transition Binaries Through Spectroscopic Morphologies


Dagny Looper, UH Institute for Astronomy
J. Davy Kirkpatrick, Caltech/IPAC
Michael Liu, UH Institute for Astronomy
Adam Burgasser, MIT
John Rayner, UH Institute for Astronomy

Brown dwarf atmospheres evolve rapidly across the L/T transition as
condensates sink below the photosphere and methane becomes the primary
carbon-bearing molecule, causing large shifts in spectral
morphologies.  This transition exhibits a peculiar brightening in
J-band known as the "J-band bump" (~T2-T4.5).  Atmospheric modelers
have been unable to reproduce the magnitude of this feature, but
binarity could be contributing to the measured overluminosity.  Indeed
a markedly higher binary fraction is found in this region.  We have
discovered 5 new T2-T4.5 dwarfs and estimate spectral types for the
two components in each system, assuming each is a binary.  By
synthesizing binary stars and their spectral composites, we examine
whether or not these early-T binaries can be distinguished from
early-T single systems based on peculiarities in spectral morphology
alone.  A necessary first step is to identify early T-dwarfs that are
indeed single.  Currently, we have obtained high resolution follow-up
with Keck LGSAO of two of these 5 new early-T dwarfs, one of which (a
composite T2.5) was resolved into a binary.  Interestingly, this pair
shows a secondary that is brighter in J, though fainter in H and Ks,
than the primary.  This binary is only the third resolved system to
show this "J-band flip" but reinforces the idea that while the large
magnitude of the "J-band bump" can perhaps be explained as an artifact
of binarity, some of the brightening is intrinsic to physical
processes occurring in the atmospheres of brown dwarfs as they cool.

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